Heating devices have been developed in thick film engineering for various applications and, in the form of coatings, are firmly bonded on the surface of a metal substrate or a steel element. In general the heating devices are constituted by electrical resistance paths and are electrically insulated from the metal substrate, i.e. metal element, by a dielectric insulating layer or by glass ceramics. Following their deposition, all strata are baked into a stratified layer which together with the steel element constitutes a compound body. Such designs are illustratively known from the German patent documents 35 36 268 A1 and 35 45 445 A1.
Problems inevitably arise if the steel element comprises a round or convex surface and must be hardened where for instance hot duct systems in injection molds are involved. As a rule said injection molds are fitted with a branched grid of feed ducts and hot duct nozzles having steel tubes which in certain applications are exposed to extremely high inner pressures. In order that the hot material in the feed or manifold system shall not cool prematurely, the said tubes are fitted peripherally with heating elements.
The PCT patent document WO 00 23 245 A1 proposes in this respect to configure the heating system in the so-called Fine Film Printing procedure wherein the individual layers are deposited using a dispenser. Such a procedure is comparatively elaborate and costly because the dispenser of the hollow dispensing needle must move in precise manner along the full surface of the ceramic, material-feed tube when depositing the insulating layer and top coat in order to make layers closed per se. As a result said layers are not always uniformly thick and/or dense, and crack formation can hardly be avoided.
Operation of the hot duct system raises another drawback: the material material feed tube is subjected at operating temperature to the pulsating internal pressure technically entailed by injection molding. Said loads applied to and heating the flow duct wall required for operating temperatures between 300 and 450° C. cause elastic expansions which are directly transmitted to the heating elements. The strata of the heating elements may rapidly enter the zone of tensile stresses, the consequences then possibly being cracks in the insulating layer, electrical shorts or even spalling of the entire heating device.
To remedy such difficulties, the heater layer already has been deposited on an accessory steel element which then is mounted on the material feed tube. Such separated heating however is devoid of any direct physical contact with the material feed tube and therefore must overcome a high thermal transfer impedance, hence incurring low heat transfer efficiency from heater elements to the tubular flow duct. This trait affects in turn the overall temperature control and the consequent cost of regulation.
The German patent document 199 41 038 A1 discloses directly depositing the heating layer on the material feed tube and to design said layer in a manner that, following baking (forming), it shall be subjected at a defined pre-compression relative to the said feed tube's wall. As a result and as a function of the elongation characteristics of the hot duct tube, a specific mismatch between the linear expansion coefficient of the glass ceramics insulating layer and the corresponding value of the metallic hot duct tube is predetermined. Such a stress tolerant connection withstands within certain limits the elastic elongations of the material feed tube. However, as regards high loads, cracks or other damages still may arise in the insulating layer.